1. Field of the Invention
The present invention relates generally to a multiband low noise amplifier, and in particular, to an input matching circuit for a multiband low noise amplifier.
2. Description of the Related Art
In a wireless communication system such as a wireless telephone, a number of electronic components for transceiving signals through a wireless medium are used. For example, a transceiver transmits and receives signals in a wireless telephone. One of the elements that is closest to a wireless interface in a signal path of the transceiver is referred to as a front end of the transceiver. At the front end of the transceiver, various components such as an antenna, a power amplifier (PA), an isolator, a low noise amplifier (LNA), and a multiplexer are arranged. Among such components, the PA or LNA includes active components and internal input and output matching circuits for controlling an input or output resistance.
A matching circuit is used to match impedances between components to avoid or reduce power loss during signal transmission. In particular, in the LNA, an impedance transfer circuit is used to maintain optimal noise impedance matching between an input signal source and an active component selected for the LNA.
Where there is the coexistence of systems following different wireless communication standards in different locations and various applications are used for such systems, there is a need for a receiver capable of operating in various frequency bands.
For example, various versions of the wireless LAN (WLAN) standards such as the Institute of Electrical and Electronic Engineers (IEEE) 802.11a, 802.11b, and 802.11g have been developed or have come into use, and the versions have different characteristics from one to another. For example, 802.11b operates in the 2.4 GHz band and provides a data rate of 11 Mbps, and 802.11a supports a data rate of 54 Mbps in the 5 GHz band, and 802.11g provides a data rate of up to 54 Mpbs in the 2.4 GHz band using Orthogonal Frequency Division Multiplexing (OFDM).
In such an environment where systems that provide different data rates in different frequency bands coexist, a terminal capable of operating in multiple bands and multiple modes is required to provide a user with high-speed data services and compatibility between systems.
A multiband, multimode transceiver is designed based on the costs, the number of additional components, and a switching mechanism upon its performance. For example, the multiband, multimode transceiver may be configured using independent wireless paths for respective frequency bands, simultaneous reception in different frequency bands, a switched inductor for selection from among the different frequency bands, and a bias current for selection from among the different frequency bands.
FIG. 1 is a block diagram of a conventional dual-band receiver. Referring to FIG. 1, signals received through two antennas pass through first band selection filters 112 and 122, respectively, are amplified by LNAs 113 and 123, pass through image reject filters 114 and 124, are selected by channel selection filters 115 and 125, and are then converted into a common intermediate frequency. In this structure, an analog-to-digital converter (ADC) 106 and a digital signal processor (DSP) 107 are shared. As a result, a number of expensive external components are required, more space is required to mount such external components, and manufacturing costs increase. Since there is a trade-off between image rejection and channel selection, double down conversion mixing is required for each path, resulting in increases in complexity and power consumption.
FIG. 2 is a block diagram of a dual-band receiver using a conventional Weaver architecture. The frequency of a first local oscillator 203 is set to an intermediate value between the bands of two duplexers 211 and 221. Band selection switches 212 and 222 that are connected to the duplexers 211 and 221, respectively, perform mode transition between two operation modes (GSM and DCS) and close a wireless path in an idle band to reduce power consumption. Outputs from the band selection switches 212 and 222 pass through band pass filters 213 and 223, and their image components are rejected and a request signal is acquired through an addition or subtraction operation of a band selection switch 205. However, the dual-band receiver using a conventional Weaver architecture does not provide sufficient image rejection and requires a large mounting space, causing an increase in manufacturing costs. Moreover, although the dual-band receiver using a conventional Weaver architecture is smaller than the dual-band receiver of FIG. 1, it consumes much power.
Unlike such dual-band receivers using two LNAs, a dual-band receiver using an LNA has been suggested in “A Compact Approach for the Design of a Dual-Bnd Low-Noise Amplifier” by Sharaf, K. M. and EIHAK, H. Y. in MWSCAS 2001, pp. 890-893. The suggested dual-band LNA is configured to include as a minimum number of components as possible. In this approach, band selection is made by three switches, and input/output matching, gains, and noise filtering are superior. However, performance degradation may occur due to noises caused by the switches and the complexity of controlling the switches and a mounting space is relatively large.
Another dual-band LNA has been suggested in “A SiGe Low Noise Amplifier for 2.4/5.2/5.7 GHz WLAN Application” by Po-Wei Lee and Hung-Wei Chiu, et al. in ISSCC 2003, pp. 264-366. In this approach, a base-emitter capacitance and a Miller capacitance function as a bias current by a base-collector capacitance and a resonance frequency for input matching can be changed by controlling an input capacitance. Such a SiGe LNA structure provides simple bias switching and requires a small mounting space. However, a change in the bias current brings about unfavorable changes in a direct current condition, leading to performance degradation.